PL SQL Interview Questions

Following are the features of PL/SQL:

• PL/SQL provides the feature of decision making, looping, and branching by making use of its procedural nature.
• Multiple queries can be processed in one block by making use of a single command using PL/SQL.
• The PL/SQL code can be reused by applications as they can be grouped and stored in databases as PL/SQL units like functions, procedures, packages, triggers, and types.
• PL/SQL supports exception handling by making use of an exception handling block.
• Along with exception handling, PL/SQL also supports error checking and validation of data before data manipulation.
• Applications developed using PL/SQL are portable across computer hardware or operating system where there is an Oracle engine.

• PL/SQL tables are nothing but objects of type tables that are modeled as database tables. They are a way to provide arrays that are nothing but temporary tables in memory for faster processing.
• These tables are useful for moving bulk data thereby simplifying the process.

• The basic structure of PL/SQL follows the BLOCK structure. Each PL/SQL code comprises SQL and PL/SQL statement that constitutes a PL/SQL block.
• Each PL/SQL block consists of 3 sections:
  • The optional Declaration Section
  • The mandatory Execution Section
  • The optional Exception handling Section

[DECLARE]
--declaration statements (optional)
BEGIN
--execution statements
[EXCEPTION]
--exception handling statements (optional)
END;

• A PL/SQL cursor is nothing but a pointer to an area of memory having SQL statements and the information of statement processing. This memory area is called a context area. This special area makes use of a special feature called cursor for the purpose of retrieving and processing more than one row.
• In short, the cursor selects multiple rows from the database and these selected rows are individually processed within a program.
• There are two types of cursors:
  • Implicit Cursor:
    • Oracle automatically creates a cursor while running any of the commands - SELECT INTO, INSERT, DELETE or UPDATE implicitly.
    • The execution cycle of these cursors is internally handled by Oracle and returns the information and status of the cursor by making use of the cursor attributes- ROWCOUNT, ISOPEN, FOUND, NOTFOUND.
  • Explicit Cursor:
    • This cursor is a SELECT statement that was declared explicitly in the declaration block.
    • The programmer has to control the execution cycle of these cursors starting from OPEN to FETCH and close.
    • The execution cycle while executing the SQL statement is defined by Oracle along with associating a cursor with it.
• Explicit Cursor Execution Cycle:
  • Due to the flexibility of defining our own execution cycle, explicit cursors are used in many instances. The following diagram represents the execution flow of an explicit cursor:

• Cursor Declaration:
  • The first step to use an explicit cursor is its declaration.
  • Declaration can be done in a package or a block.
  • Syntax: CURSOR cursor_name IS query; where cursor_name is the name of the cursor, the query is the query to fetch data from any table.
• Open Cursor:
  • Before the process of fetching rows from cursor, the cursor has to be opened.
  • Syntax to open a cursor: OPEN cursor_name;
  • When the cursor is opened, the query and the bind variables are parsed by Oracle and the SQL statements are executed.
  • The execution plan is determined by Oracle and the result set is determined after associating the cursor parameters and host variables and post these, the cursor is set to point at the first row of the result set.
• Fetch from cursor:
  • FETCH statement is used to place the content of the current row into variables.
  • Syntax: FETCH cursor_name INTO variable_list;
  • In order to get all the rows of a result set, each row needs to be fetched.
• Close Cursor:
  • Once all the rows are fetched, the cursor needs to be closed using the CLOSE statement.
  • Syntax: CLOSE cursor_name;
  • The instructions tell Oracle to release the memory allocated to the cursor.
    • Cursors declared in procedures or anonymous blocks are by default closed post their execution.
    • Cursors declared in packages need to be closed explicitly as the scope is global.
    • Closing a cursor that is not opened will result in INVALID_CURSOR exception.

• We use this clause while referencing the current row from an explicit cursor. This clause allows applying updates and deletion of the row currently under consideration without explicitly referencing the row ID.
• Syntax:
  UPDATE table_name SET field=new_value WHERE CURRENT OF cursor_name

• This can be done by using Pragma called EXCEPTION_INIT.
• This gives the flexibility to the programmer to instruct the compiler to provide custom error messages based on the business logic by overriding the pre-defined messages during the compilation time.
• Syntax:

DECLARE 
  exception_name EXCEPTION; 
  PRAGMA EXCEPTION_INIT (exception_name, error_code); 
BEGIN 
// PL/SQL Logic
EXCEPTION
 WHEN exception_name THEN
    // Steps to handle exception
END;

• As the name indicates, ‘Trigger’ means to ‘activate’ something. In the case of PL/SQL, a trigger is a stored procedure that specifies what action has to be taken by the database when an event related to the database is performed.

• Syntax:

TRIGGER trigger_name
 trigger_event
 [ restrictions ]
BEGIN
 actions_of_trigger;
END;

In the above syntax, if the trigger_name the trigger is in the enabled state, the trigger_event causes the database to fire actions_of_trigger if the restrictions are TRUE or unavailable.

• They are mainly used in the following scenarios:
  • In order to maintain complex integrity constraints.
  • For the purpose of auditing any table information.
  • Whenever changes are done to a table, if we need to signal other actions upon completion of the change, then we use triggers.
  • In order to enforce complex rules of business.
  • It can also be used to prevent invalid transactions.
• You can refer https://docs.oracle.com/database/121/TDDDG/tdddg_triggers.htm for more information regarding triggers.

• This statement is used by anonymous blocks of PL/SQL such as non-stored and stand-alone procedures. When they are being used, the statement should come first in the stand-alone file.

• Comments are those sentences that have no effect on the functionality and are used for the purpose of enhancing the readability of the code. They are of two types:
  • Single Line Comment: This can be created by using the symbol -- and writing what we want to mention as a comment next to it.
  • Multi-Line comment: These are the comments that can be specified over multiple lines and the syntax goes like /* comment information */
• Example:

SET SERVEROUTPUT ON; 
DECLARE 
 
-- Hi There! I am a single line comment.
var_name varchar2(40) := 'I love PL/SQL' ;  
BEGIN 
 /* 
 Hi! I am a multi line
 comment. I span across
 multiple lines
 */
dbms_output.put_line(var_name);
END; 
/
Output:
I love PL/SQL

• WHEN clause specifies for what condition the trigger has to be triggered.

The PL/SQL engine does the process of compilation and execution of the PL/SQL blocks and programs and can only work if it is installed on an Oracle server or any application tool that supports Oracle such as Oracle Forms.

• PL/SQL is one of the parts of Oracle RDBMS, and it is important to know that most of the Oracle applications are developed using the client-server architecture. The Oracle database forms the server-side and requests to the database form a part of the client-side.
• So based on the above fact and the fact that PL/SQL is not a standalone programming language, we must realize that the PL/SQL engine can reside in either the client environment or the server environment. This makes it easy to move PL/SQL modules and sub-programs between server-side and client-side applications.
• Based on the architecture shown below, we can understand that PL/SQL engine plays an important role in the process and execute the PL/SQL statements and whenever it encounters the SQL statements, they are sent to the SQL Statement Processor.

• Case 1: PL/SQL engine is on the server: In this case, the whole PL/SQL block gets passed to the PL/SQL engine present on the Oracle server which is then processed and the response is sent.
• Case 2: PL/SQL engine is on the client: Here the engine lies within the Oracle Developer tools and the processing of the PL/SQL statements is done on the client-side.
  • In case, there are any SQL statements in the PL/SQL block, then they are sent to the Oracle server for SQL processing.
  • When there are no SQL statements, then the whole block processing occurs at the client-side.

• SYSDATE:
  • This keyword returns the current time and date on the local database server.
  • The syntax is SYSDATE.
  • In order to extract part of the date, we use the TO_CHAR function on SYSDATE and specify the format we need.
  • Usage:
    • SELECT SYSDATE FROM dual;
    • SELECT id, TO_CHAR(SYSDATE, 'yyyy/mm/dd') from InterviewBitEmployeeTable where customer_id < 200;
• USER:
  • This keyword returns the user id of the current session.
  • Usage:
    • SELECT USER FROM dual;

• %TYPE: This declaration is used for the purpose of anchoring by providing the data type of any variable, column, or constant. It is useful during the declaration of a variable that has the same data type as that of its table column.
  • Consider the example of declaring a variable named ib_employeeid which has the data type and its size same as that of the column employeeid in table ib_employee. 
    The syntax would be : ib_employeeid ib_employee.employeeid%TYPE;
• %ROWTYPE: This is used for declaring a variable that has the same data type and size as that of a row in the table. The row of a table is called a record and its fields would have the same data types and names as the columns defined in the table.
  • For example: In order to declare a record named ib_emprecord for storing an entire row in a table called ib_employee, the syntax is:
    ib_emprecord ib_employee%ROWTYPE;

• The functions that are used for manipulating the character data are called String Functions.
  • LEFT: This function returns the specified number of characters from the left part of a string.
    • Syntax: LEFT(string_value, numberOfCharacters).
    • For example, LEFT(‘InterviewBit’, 9) will return ‘Interview’.
  • RIGHT: This function returns the defined number of characters from the right part of a string.
    • Syntax: RIGHT(string_value, numberOfCharacters)
    • For example, RIGHT(‘InterviewBit’,3) would return ‘Bit’.
  • SUBSTRING: This function would select the data from a specified start position through the number of characters defined from any part of the string.
    • Syntax: SUBSTRING(string_value, start_position, numberOfCharacters)
    • For example, SUBSTRING(‘InterviewBit’,2,4) would return ‘terv’.
  • LTRIM: This function would trim all the white spaces on the left part of the string.
    • Syntax: LTRIM(string_value)
    • For example, LTRIM(’ InterviewBit’) will return ‘InterviewBit’.
  • RTRIM: This function would trim all the white spaces on the right part of the string.
    • Syntax: RTRIM(string_value)
    • For example, RTRIM('InterviewBit ') will return ‘InterviewBit’.
  • UPPER: This function is used for converting all the characters to the upper case in a string.
    • Syntax: UPPER(string_variable)
    • For example, UPPER(‘interviewBit’) would return ‘INTERVIEWBIT’.
  • LOWER: This function is used for converting all the characters of a string to lowercase.
    • Syntax: LOWER(string_variable)
    • For example, LOWER(‘INterviewBit’) would return ‘interviewbit’.

• ROLLBACK command is used for rolling back all the changes from the beginning of the transaction.
• ROLLBACK TO command is used for undoing the transaction only till a SAVEPOINT. The transactions cannot be rolled back before the SAVEPOINT and hence the transaction remains active even before the command is specified.

• SYS.ALL_DEPENDENCIES is used for describing all the dependencies between procedures, packages, triggers, functions that are accessible to the current user. It returns the columns like name, dependency_type, type, referenced_owner etc.

• The THEN and NOW tables are the virtual tables that are available during the database trigger execution. The table columns are referred to as THEN.column and NOW.column respectively.
• Only the NOW.column is available for insert-related triggers.
• Only the THEN.column values are available for the DELETE-related triggers.
• Both the virtual table columns are available for UPDATE triggers.

• The cursors that are declared in the procedures will have the local scope and hence they cannot be used by other procedures.
• The cursors that are declared in package specifications are treated with global scope and hence they can be used and accessed by other procedures.

• These are the three transaction specifications that are available in PL/SQL.
• COMMIT: Whenever any DML operations are performed, the data gets manipulated only in the database buffer and not the actual database. In order to save these DML transactions to the database, there is a need to COMMIT these transactions.
  • COMMIT transaction action does saving of all the outstanding changes since the last commit and the below steps take place:
    • The release of affected rows.
    • The transaction is marked as complete.
    • The details of the transaction would be stored in the data dictionary.
  • Syntax: COMMIT;
• ROLLBACK: In order to undo or erase the changes that were done in the current transaction, the changes need to be rolled back. ROLLBACK statement erases all the changes since the last COMMIT.
  • Syntax: ROLLBACK;
• SAVEPOINT: This statement gives the name and defines a point in the current transaction process where any changes occurring before that SAVEPOINT would be preserved whereas all the changes after that point would be released.
  • Syntax: SAVEPOINT <savepoint_name>;

• We can use DBMS_OUTPUT and DBMS_DEBUG statements for debugging our code:
  • DBMS_OUTPUT prints the output to the standard console.
  • DBMS_DEBUG prints the output to the log file.

• A table that is being modified by the usage of the DML statement currently is known as a mutating table. It can also be a table that has triggers defined on it.
• A table used for reading for the purpose of referential integrity constraint is called a constraining table.

• The outcomes of the execution of the DML statement is saved in the following 4 cursor attributes:
  • SQL%FOUND: This returns TRUE if at least one row has been processed.
  • SQL%NOTFOUND: This returns TRUE if no rows were processed.
  • SQL%ISOPEN: This checks whether the cursor is open or not and returns TRUE if open.
  • SQL%ROWCOUNT: This returns the number of rows processed by the DML statement.

• Yes, these type of declarations are possible.
• Number (9, 12) indicates that there are 12 digits after decimal point. But since the maximum precision is 9, the rest are 0 padded like 0.000999999999.
• Number (9, -12) indicates there are 21 digits before the decimal point and out of that there are 9 possible digits and the rest are 0 padded like 999999999000000000000.0

DECLARE
   n NUMBER;
   average NUMBER :=0 ;
   sum NUMBER :=0 ;
   count NUMBER :=0 ;
BEGIN
   -- Take input from user
   n := &input_number;
   WHILE(n<>0)
       LOOP
           -- Increment count to find total elements
           count := count+1;
           -- Sum of elements entered
           sum := sum+n;
           -- Take input from user
           n := &input_number;
       END LOOP;
   -- Average calculation
   average := sum/count;
   DBMS_OUTPUT.PUT_LINE(‘Average of entered numbers is ’||average);
END;

• Consider that we are fetching details of employees from ib_employee table where salary is a parameter for filter.

CREATE PROCEDURE get_employee_details @salary nvarchar(30)
AS
BEGIN
   SELECT * FROM ib_employee WHERE salary = @salary;
END;

--declare 2 dates of type Date
DECLARE
   start_date Date; 
   end_date Date; 
   sundays_count Number:=0; 
BEGIN
   -- input 2 dates
   start_date:='&input_start_date'; 
   end_date:='&input_end_date';
   /* 
   Returns the date of the first day after the mentioned date 
   and matching the day specified in second parameter.
   */
   start_date:=NEXT_DAY(start_date-1, 'SUNDAY');
   --check the condition of dates by using while loop. 
   while(start_date<=end_date) 
   LOOP 
       sundays_count:=sundays_count+1; 
       start_date:=start_date+7; 
   END LOOP; 
   
   -- print the count of sundays
   dbms_output.put_line('Total number of Sundays between the two dates:'||sundays_count); 
END; 
/ 

Input:
start_date = ‘01-SEP-19’
end_date = ‘29-SEP-19’

Output:
Total number of Sundays between the two dates: 5

DECLARE
 employee_salary  NUMBER(8,2);

 PROCEDURE update_salary (
   emp        NUMBER, 
   salary IN OUT NUMBER
 ) IS
 BEGIN
   salary := salary + 1000;
 END;

BEGIN
 SELECT salary INTO employee_salary
 FROM ib_employee
 WHERE employee_id = 102;

 DBMS_OUTPUT.PUT_LINE
  ('Before update_salary procedure, salary is: ' || employee_salary);

 update_salary (100, employee_salary);

 DBMS_OUTPUT.PUT_LINE
  ('After update_salary procedure, salary is: ' || employee_salary);
END;
/

Result:

Before update_salary procedure, salary is: 17000
After update_salary procedure, salary is: 18000

DECLARE
-- Declared variables string, letter, reverse_string where string is the original string. 
string VARCHAR2(10) := 'abccba'; 
letter VARCHAR2(20); 
reverse_string VARCHAR2(10); 
BEGIN
FOR i IN REVERSE 1..LENGTH(string) LOOP 
 letter := SUBSTR(string, i, 1);
 -- concatenate letter to reverse_string variable
 reverse_string := reverse_string ||''||letter; 
END LOOP;
IF reverse_string = string THEN
       dbms_output.Put_line(reverse_string||''||' is palindrome'); 
ELSE
       dbms_output.Put_line(reverse_string ||'' ||' is not palindrome'); 
END IF; 
END; 

DECLARE
-- declare necessary variables 
   -- num represents the given number
   -- number_to_word represents the word format of the number
   -- str, len and digit are the intermediate variables used for program execution
num   INTEGER; 
number_to_word VARCHAR2(100); 
digit_str   VARCHAR2(100); 
len   INTEGER; 
digit   INTEGER; 
BEGIN
   num := 123456; 
   len := LENGTH(num); 
   dbms_output.PUT_LINE('Input: ' ||num);
   -- Iterate through the number one by one
   FOR i IN 1..len LOOP 
       digit := SUBSTR(num, i, 1);
       -- Using DECODE, get the str representation of the digit
       SELECT Decode(digit, 0, 'Zero ', 
                       1, 'One ', 
                       2, 'Two ', 
                       3, 'Three ', 
                       4, 'Four ', 
                       5, 'Five ', 
                       6, 'Six ', 
                       7, 'Seven ', 
                       8, 'Eight ', 
                       9, 'Nine ') 
       INTO digit_str 
       FROM dual;
       -- Append the str representation of digit to final result.
       number_to_word := number_to_word || digit_str; 
   END LOOP;
   dbms_output.PUT_LINE('Output: ' ||number_to_word); 
END; 

Input: 12345
Output: One Two Three Four Five

DECLARE
--Declare variables num, sum_of_digits and remainder of datatype Integer 
num  INTEGER; 
sum_of_digits INTEGER; 
remainder  INTEGER; 
BEGIN
num := 123456; 
sum_of_digits := 0;
-- Find the sum of digits until original number doesnt become null 
WHILE num <> 0 LOOP 
 remainder := MOD(num, 10); 
 sum_of_digits := sum_of_digits + remainder; 
 num := TRUNC(num / 10); 
END LOOP; 
dbms_output.PUT_LINE('Sum of digits is '|| sum_of_digits); 
END; 

Input: 9874
Output: 28

• PL/SQL is a programming extension of SQL developed by Oracle which combines the power of SQL in the field of data manipulation and the power of procedural language for faster and effective processing of data thereby resulting in the creation of powerful queries.
• PL/SQL enhances the security, increases the platform portability, and makes it more robust by means of instructing the compiler ‘what to do’ and ‘how to do’ using SQL and procedural form respectively.
• Finally, it gives more power over the database to the programmers due to the feature of decision making, filtering, and looping abilities thereby making it a more convenient and reliable means for the programmers to work on it.

References:
https://oracle-base.com/articles/misc/introduction-to-plsql

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